Since only a portion of the oil contained in a petroleum reservoir can be recovered by primary methods, it has become conventional practice to employ various secondary and tertiary recovery techniques to produce additional quantities of oil. Of the various secondary and tertiary methods available, one of the most widely practiced techniques is the displacement of oil from the reservoir with a driving fluid such as floodwater injected for that purpose. Normally in carrying out the flooding process, a series of input wells spaced apart from one or more producing wells are drilled into and opened to the oil-producing strata. Aqueous drive fluid, such as water, brine, or a viscous aqueous polymer solution, is injected into the reservoir under pressure, forcing some of the oil towards the producing well or wells.
While water flooding has been rather widely practiced in recent years, it is not without considerable operating problems and economic limitations, particularly those associated with low oil recoveries in proportion to the amount of water injected. Various surfactant and solvent floods have been proposed as means for recovering additional quantities of oil over that recoverable by conventional water flooding. Synthetic surfactants are expensive and the total amount of synthetic surfactant employed in a given waterflood operation is generally quite high because of the very large total amount of water which has to be introduced into a given partially depleted oil-bearing formation treated by the waterflood technique. This raises the cost of recovery to a figure which is sometimes exorbitant and frequently uneconomical because of the relatively high price of the surfactant, particularly when the total cost of the water-flood operation is compared with the amount of oil recovered by the use of this technique. On the other hand, crude petroleum is known to contain varying amounts of surfactant-yielding materials. These have generally been thought of as being simply saponifiable materials such as petroleum acids which can react with alkaline materials to form soaps that reduce the interfacial tension between the crude petroleum and water. Accordingly, the use of alkaline waterflooding, to activate such surfactants as a tertiary recovery mechanism, has been extensively investigated. Seifert and Howells, Anal. Chem. 41 (4), 554 (1969) persistently fractionated the many acidic components of a California crude oil into minute quantities, by a sequence of exhaustive back extractions, in an effort to ascertain their interfacial activities. However, none of the fractions isolated revealed surface tensions lower than that of the original crude oil except when measurement was highly dependent upon the alkaline concentration, so narrow a range of pH as to preclude any given practical use. A similar phenomenon is experienced with crude oils having a significantly high content of saponifiable material: low interfacial tension is exhibited primarily at a narrow pH range resulting to tolerances for alkaline flooding which are difficult to achieve.
It is now apparent that the alkaline water-flooding method for enhanced oil recovery is a complex process. Investigators have identified compounds other than carboxylic acids, such as phenols or porphyrins, as beneficial to oil recovery due to the low interfacial tensions they exhibit. Johnson, J. Petrl. Tech. 85 (January 1976), has outlined requirements other than ultralow surface tension to insure efficient, stable recovery of a given oil in a given reservoir, e.g., spontaneous emulsification, entrainment, entrapment, wettability reversal in both directions, and the like. Even with petroleum acids, the mechanism is not entirely clear. For example, it would seem evident that a higher acid number of a given crude oil, either by nature or due to the addition of a known acid, would lower its interfacial tension. However, Cooke, Williams, and Kolodzie, J. Petrl. Tech. 26 (12), 1365 (1974) found that though in-situ oxidation with air further increases the acid number of a given crude oil, this artificially created high-acid-number crude oil could not successfully be flooded with alkaline water.
The present invention provides means for tertiary oil recovery useful over a relatively wide pH range and which is economical and flexible. Specifically, the invention proceeds by isolating the native petroleum fractions responsible for lowering the interfacial tension of petroleum in aqueous alkaline systems. The fractions are isolated from petroleum of the type having reduced interfacial tension with water at high pH, for example, 2-30 dynes/cm., or less. A non-aqueous slug is formed by subjecting petroleum of such type to fractionation by distillation including distillation under vacuum, e.g. 1 mm of Hg (1Torr), in a certain temperature range and recovering a surfactant fraction of the petroleum distilled in that temperature range. The temperature range critically is 100.degree.-200.degree. C. under a vacuum of 1 Torr, or equivalent pressure and temperature. It is believed that such distillation results in separation of the desired petroleum fraction with no or minimum destruction of the structure of the molecules constituting the fraction. The fraction forms a surfactant when contacted with aqueous alkaline, its surfactant character increasing rapidly with time upon such contact and thereafter decreasing. Accordingly, the fraction can be injected as a slug into the reservoir, followed by alkaline waterflooding. The fraction can itself constitute the slug or can be mixed with petroleum to from a slug (for example, at a level of one part fraction to 10 to 10.sup.4 parts of petroleum). The petroleum can be obtained directly from that reservoir.
In a further embodiment, the fraction is further concentrated by removing benzene-soluble components to obtain as a surfactant portion of said fraction an ether-soluble residue having a density greater than water. Whereas the fraction itself can constitute, as an example, about 30% of the petroleum from which it is derived, the ether-soluble residue constitutes, again by way of example, about 10% of the fraction. The residue, i.e., concentrate, can under certain circumstances be used itself as a slug, or it can be mixed with petroleum to form a slug (for example, at a level of one part concentrate to 10 to 10.sup.6 parts petroleum) or can be mixed with the fraction itself (for example, at a level of one part concentrate to 1 to 10.sup.3 parts fraction) to provide an enriched fraction, and either form can be used as a slug as above.
It will be appreciated that the fraction separated by the foregoing method comprises natural components of the petroleum and this gives rise to a significant advantage enabling the fraction or residue concentrate to be used for surfactant purposes other then tertiary oil recovery. Surfactants are used to prevent or treat certain types of well formation damage, but if the wrong surfactant is used, damage can be caused or aggravated. For example, such damage as oil-wetting of formation rock, interfacial film or membrane blocks, and particle blocks due to floculation can all be caused or aggravated by using the wrong surfactant for a particular well. The native petroleum fraction cannot suffer such drawbacks because it is native to the petroleum itself. The fraction can be stored as such, or reacted with alkali to form an anionic surfactant material and then stored. Subsequently, the surfactant can be used for well work.
It is believed that neither the fraction nor the concentrate therefrom have been previously isolated as such, and hence constitute new compositions of matter, as do the mixtures with petroleum and as does the fraction enriched with the concentrate. While it is not desired to be limited to any particular theory, it is believed that the fraction and concentrate contain hydrogen displaceable components including one, two, three or four of the following component types: EQU R--SH ##STR1## EQU R--OH ##STR2## in which R is a hydrocarbon of 3 to 20 carbon atoms, R.sub.1 is hydrogen or a hydrocarbon of 3 to 20 carbon atoms, and R and R.sub.1 are either separate or cyclicallycombined.
Injection of the slug proceeds in accordance with known methods of slug injection and is followed by alkaline waterflooding as known to the art. Preferably the alkalinity is at least 10.9 pH and the floodwater preferably contains at least 100 ppm. of a halide salt such as sodium chloride. The salt serves to act as a common ion preventing the removal of alkali by precipitation in the pesence of divalent ion such as calcium or magnesium. It is a characteristic of the native surfactant fraction that the optimum interfacial tension extends throughout a relatively broad pH range. By way of example, a particular native surfactant fraction can have an optimum interfacial tension when measured in water, containing 7500 ppm NaCl, of less than 0.03 dynes/cm. throughout a pH range of 0.5 pH units or more; the crude from which it is obtained exhibits that level of interfacial tension over a narrower pH range of approximately 0.2 pH units.
It will be appreciated that the present invention can be practiced at the site of the reservoir itself. Crude petroleum recovered from the reservoir by primary, secondary, or tertiary methods can be subjected to fractional distillation under the conditions outlined above to obtain the native surfactant fraction. Where the reservoir has been subjected to alkaline flooding, the recovered oil can be separated from the alkaline solution and the oil subjected to distillation. Optionally one can further separate the aforementioned residue, for example, by elution, to obtain the surfactant concentrate. The alkaline water can then be recycled for further flooding after the injection of a slug comprising the surfactant.
It will also be appreciated that the method described above for separation of native surfactant by fractional distillation can be used to determine the amenability of a particular subterranean reservoir to recovery by alkaline waterflooding. The extent to which alkaline waterflooding can be effective can be determined by measuring the degree of reduction in interfacial tension exhibited by the native petroleum fraction. The undertaking of an alkaline waterflooding process involves tremendous capital investment, manpower, and reagent commitment; such an amenability determination can provide the basis for determining whether such commitment is warranted.
Other advantages and details of the invention will become apparent in accordance with the following description.